A device (to be referred to as a micro-electromechanical system [MEMS] variable capacitance device hereinafter) in which a MEMS is applied as a variable capacitance element can achieve low loss, high isolation, and high linearity, and hence is expected as a key device for implementing a multi-band, multi-mode configuration of a next generation portable terminal.
The MEMS variable capacitance device can achieve low loss because a low-resistivity metal material is used as the capacitance electrode of a variable capacitance element as a movable portion. More specifically, aluminum (Al) or gold (Au) is used as the capacitance electrode. A metal material like this is generally ductile.
When the movable capacitance electrode is repetitively driven, therefore, the shape of the movable portion is distorted because of creep (a shape change caused by stress) of a member forming the movable portion.
In the MEMS variable capacitance device, if the electrode as a movable portion deforms because of the creep, the spacing between two electrodes forming the capacitance electrode changes from a predetermined spacing. Accordingly, the capacitance of the variable capacitance element changes from a designed value when the MEMS variable capacitance device is used for long time periods.
As a means for avoiding this creep, a material having small plastic deformation, for example, a hard metal material having brittleness such as an aluminum titanium (AlTi) alloy or tungsten (W) is used as the movable portion. However, a metal material like this has high resistivity and increases the loss, when compared to Al and Au.
As another means, a structure obtained by stacking a metal layer of a ductile material and an insulating layer of a brittle material is used as the movable portion. In this multilayered structure, however, the variable portion readily warps because of the difference between the internal stresses of the ductile material and brittle material. Consequently, the shape of the movable portion deforms.
To solve the problem of the deterioration of the movable portion caused by the creep and the problem of the deformation of the movable portion caused by the internal stress as described above, a method of connecting the movable portion and an anchor portion by using a spring portion made of the ductile material and having a small spring constant and a spring portion made of the brittle material and having a large spring constant has been proposed.
Unfortunately, it is difficult to form a structure having desired characteristics when the spring portion is formed using the brittle material.